Regioselective Isomerization of Terminal Alkenes Catalyzed by a PC(sp3)Pincer Complex with a Hemilabile Pendant Arm

Article abstract of DOI:10.1002/cctc.202001308

We describe an efficient protocol for the regioselective isomerization of terminal alkenes employing a previously described bifunctional Ir-based PC(sp³)complex (4) possessing a hemilabile sidearm. The isomerization, catalyzed by 4, results in a one-step shift of the double bond in good to excellent selectivity, and good yield. Our mechanistic studies revealed that the reaction is driven by the stepwise migratory insertion of Ir?H species into the terminal double bond/β-H elimination events. However, the selectivity of the reaction is controlled by dissociation of the hemilabile sidearm, which acts as a selector, favoring less sterically hindered substrates such as terminal alkenes; importantly, it prevents recombination and further isomerization of the internal ones.

Full text of DOI:10.1002/cctc.202001308

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Regioselective Isomerization of Terminal Alkenes Catalyzed
3
by a PC(sp )Pincer Complex with a Hemilabile Pendant Arm
[a]
[b]
[b]
[b]
Sophie De-Botton, D.Sc. Oleg A. Filippov, Elena S. Shubina, Natalia V. Belkova,* and
Dmitri Gelman*
[
a, c]
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We describe an efficient protocol for the regioselective isomer-
ization of terminal alkenes employing a previously described
bifunctional Ir-based PC(sp )complex (4) possessing a hemilabile
sidearm. The isomerization, catalyzed by 4, results in a one-step
shift of the double bond in good to excellent selectivity, and
good yield. Our mechanistic studies revealed that the reaction
is driven by the stepwise migratory insertion of IrÀ H species
into the terminal double bond/β-H elimination events. How-
ever, the selectivity of the reaction is controlled by dissociation
of the hemilabile sidearm, which acts as a selector, favoring less
sterically hindered substrates such as terminal alkenes; impor-
tantly, it prevents recombination and further isomerization of
the internal ones.
3
Introduction
namically via conjugation to certain functional groups or
sterically by skeleton branching, can be achieved, a more
general approach would rely on controlling by using a tunable
catalyst. For example, one of the more interesting catalyst
design principles that have been recently employed to achieve
high selectivity and reactivity control relies on the ligand-metal
The double bond is one of the most versatile functional groups
in organic chemistry because it allows facile and selective
manipulation of the molecular skeleton. In general, isomer-
ization of the readily available terminal alkenes into an array of
structurally more complex isomers is a useful strategy to create
molecular diversity. Therefore, changing the position of a
double bond in a controllable fashion is a very important tool
in organic synthesis, especially for designing one-pot tandem
transformations in conjunction with mechanistically related
[12]
interplay with the aid of a pendant functional group.
[
1]
Our group explores a family of dibenzobarrelene-based
pincer ligand systems for various applications in homogeneous
[13]
catalysis directed toward organic synthesis. This ligand class
offers a modular, yet structurally and conformationally stable
3
transformations
such
as
hydroformylation,
meth-
platform for the synthesis of robust C(sp )-metalated
[14]
oxycarbonylation, hydrocyanation, hydroboration, hydrosilyla-
tion, olefin metathesis, conjugate additions and many others,
affording the synthesis of fine and commodity chemicals, as
compounds
bearing various functional groups (Figure 1).
These functional groups are not innocent in catalysis and are
often responsible for secondary catalyst-substrate interactions
[2]
[15]
well as smart utilization of petroleum feedstock (Scheme 1).
or even ligand-metal cooperative bond activation.
A wide range of isomerization catalysts have been devel-
oped based on transition-metal complexes of such metals as
Recently, we demonstrated that the presence of a hemi-
labile coordinating site in a ligand may lead to a marked
change in the organometallic behavior and the catalytic activity
of the corresponding transition metal complex, for example,
significantly influencing the reactivity of incoming substrates
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Ir, Pd, Ru, Co, Fe, Rh, Mo, Ni, and more. However,
controlling regio- and stereoselectivities in such an isomer-
ization has been a challenge so far because a mixture of
positional and geometric isomers is produced during the
reaction. Although the site selectivity, controlled thermody-
[16]
and the rates of elementary reactions. For example, our brief
comparative studies aiming to identify the reactive species in
the transfer dehydrogenation of alkanes demonstrated a
significant isomerization of linear alkenes accompanying that
[a] S. De-Botton, Prof. D. Gelman
[17]
major reaction.
Institute of Chemistry, Edmond J. Safra Campus
The Hebrew University of Jerusalem, Jerusalem, 91904 (Israel)
E-mail: dmitri.gelman@mail.huji.ac.il
[b] D.S. O. A. Filippov, Prof. E. S. Shubina, Prof. N. V. Belkova
A.N. Nesmeyanov Institute of Organoelement Compounds
Russian Academy of Sciences
Vavilov Street 28, 119991 Moscow (Russia)
E-mail: nataliabelk@ineos.ac.ru
c] Prof. D. Gelman
[
Peoples’ Friendship University of Russia (RUDN University)
Miklukho-Maklay St., 6, 117198 Moscow (Russia)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.202001308
This publication is part of a joint Special Collection with EurJIC on “Pincer
Chemistry & Catalysis”. Please follow the link for more articles in the col-
lection.
3
Figure 1. Previously reported bifunctional C(sp )-metalated compounds.
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Scheme 1. Isomerization of terminal olefins.
Therefore, in this work, we were interested in examining
this unwanted transformation in more detail in order to convert
it into a useful synthetic method by means of our highly
tunable and modular bifunctional catalysts.
functionless reference with pincer ligand geometry similar to
that of 1–3 (Figure 2).
The initial optimization studies were performed under
reaction conditions similar to the transfer dehydrogenation
experiments; they comprised 1-octene, 0.5 mol% of catalysts 1–
1
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5
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5
5
5
5
5
5
5
, and 1 mol% of t-BuONa base in non-polar 1,3,5-mesitylene
Results and discussion
under high temperature conditions. We decided to stick to
these conditions because according to our initial observations,
the mechanistically related hydrogen transfer and isomerization
are catalyzed by a common species. Under these conditions,
high activity was achieved only by the flexible catalyst 4, which
drove the reaction to complete conversion after 24 hours at 120
Catalysis. Initial alkene isomerization studies were conducted
using five complexes reported by us in the past as catalysts for
the transfer dehydrogenation of alkanes.
possess hemilabile coordinating alkoxyl groups: 1–3 are charac-
[
17a,b]
Compounds 1–4
3
0
terized by mer-configuration of the PC(sp )ligand and a rigid
C (entries 1–5, Table 1). Interestingly, our attempt to improve
coordination of the sidearm having a different steric bulk
complex, whereas 4 is a more flexible analogue with fac-
coordination of the ligand. Complex 5 was used as a
the performance of the catalyst by replacing sodium t-butoxide
with another base (e.g., Cs CO , DBU, LiHMDS, and i-Pr NEt)
2
3
2
eventually led us to conclude that bases are superfluous for this
catalyst and that complete conversions can be achieved under
base-free conditions (entry 6, Table 1). Lowering reaction tem-
0
peratures to 80–100 C diminished the conversions significantly
down to 35–77%, respectively (entries 7–9, Table 1).
Substantial progress was achieved upon screening different
solvents. A series of experiments showed that, whereas strongly
coordinating solvents, such as acetonitrile or propionitrile,
inhibit isomerization at lower temperatures, weakly coordinat-
ing solvents, such as DME or THF, accelerate it in comparison
with the nonpolar solvents (entries 8–14, Table 1); a reaction in
0
MeÀ THF led to almost full conversion of 1-octene at 80 C.
3
Figure 2. Bifunctional C(sp )-metalated compounds used for catalysis.
Table 1. Initial optimization studies.
[
e]
[f]
Entry
Substrate
T [°C]
Solvent
Cat.
C/S/b
t [h]
Conv. [%]
[
[
[
a]
a]
a]
1
2
3
4
5
6
7
8
9
1
1
1
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
1-octene
120
120
120
120
120
120
100
80
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Benzene
Acetonitrile
Propionitrile
DME
THF
MeÀ THF
1
2
3
4
5
4
4
4
4
4
4
4
4
4
1:200:2
1:200:2
1:200:2
1:200:2
1:200:2
1:200:0
1:200:0
1:200:0
1:200:0
1:200:0
1:200:0
1:200:0
1:200:0
1:200:0
24
24
24
24
24
24
24
24
24
24
24
24
24
24
8
0
6
[a]
>99
17
>99
77
35
41
<1
<1
51
53
95
[
[
[
a]
b]
c]
[d]
[
d]
[
80
80
80
80
80
80
d]
d]
0
1
2
[
[d]
[
[
d]
d]
13
1
4
À 3
À 3
Conditions: [a] Reaction conditions: alkene (0.28 mmol), catalyst 1–5 (1.4*10 mmol), base (2.8*10 mmol) in 1 mL of mesitylene at 120°C; [b] Reaction
À 3
conditions: alkene (0.28 mmol), catalyst 1–5 (1.4*10 mmol), no base, in 1 mL of mesitylene at 120°C; [c] Reaction conditions: alkene (0.28 mmol), catalyst 1–
À 3
À 3
5
(1.4*10 mmol), no base, in 1 mL of mesitylene at 100°C; [d] Reaction conditions: alkene (0.28 mmol), catalyst 1–5 (1.4*10 mmol), no base, in 1 mL of
different solvents at 80°C; [e] C/S/b=Catalyst/Substrate/base ratio; [f] Conversion was determined by NMR spectroscopy using internal standard.
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With the optimized reaction conditions in hand, the
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substrate scope for the isomerization was investigated. As
shown in Table 2, the isomerization process can be extended to
a wide variety of terminal alkenes. All the reactions were carried
out using two solvents: (1) MeÀ THF, since this solvent resulted
in the highest rates (vide supra), and (2) benzene, since this
resulted in the highest selectivity (vide infra). Product composi-
tions and stereochemistry were determined by GC or H-NMR.
Scheme 2. Isomerization of terminal olefins.
For example, isomerization of allyl benzene was complete after
0
2
4 hours of heating at 80 C in the presence of 0.5 mol% of 4 in
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
MeTHF; there was an excellent E/Z selectivity of 20:1 (entries 1–
, Table 2). The reaction in benzene, however, reached only
4% conversion with the same selectivity. No rate inhibition
was observed even with coordinating substrates such as
eugenol and complete conversions of the starting material
predominantly into the thermodynamically stable E-isomer
were recorded in both the polar and non-polar media
2
7
reaction gives two possible metal-alkyl intermediates capable of
β-H elimination into either the starting material or its double-
bond-shifted isomer upon the regeneration of the reactive
catalyst in the form of metal hydride. The π-allyl mechanism (B)
involves the oxidative addition of low-valent metal across the
allylic CÀ H bond of the pre-coordinated alkene. The resultant π-
allyl complex may reductively eliminate the isomerized
(
entries 3–4, Table 2).
1
[20]
However, careful H-NMR analysis of the mixture from a
product.
reaction of a longer chain aliphatic substrate, 1-octene, revealed
a good regioselectivity for 2-octenes – 58% of 2-octene,
although with a relatively low stereoselectivity – E/Z mixture
The virtue of the base-free reaction promoted by catalyst 4
may, in principle, indicate that the reaction takes place via the
metal-alkyl pathway because the formation of the low-valent Ir
[21]
(
2:1) after 61% conversion. Other octene isomers were
(I) species without base assistance is rather unlikely.
observed in smaller quantities up to 3% total for 3- and 4-
octenes; this corresponds to a ratio of 19:1 between the 2-
octene and the higher regioisomers. Interestingly, analysis of
the same reaction performed in MeÀ THF revealed a diminished
Nevertheless, we carried out a stoichiometric H/D scram-
bling experiment to clarify the mechanism underlying the
reaction. A sample of allylbenzene deuterated exclusively in the
benzylic position was prepared. Monitoring its isomerization in
the presence of stoichiometric 4 in benzene at 80°C over the
5
:1 regioselectivity toward 2-octene (2:1 E/Z ratio), along with
2
other isomers that were present in the reaction mixture after
5% conversion (entries 5–6, Table 2). Similar results have been
course of 12 h by D NMR revealed that the peak corresponding
9
to d -allyl benzene at 3.2 ppm disappeared, and that the
2
obtained using longer-chained alkenes, such as 1-dodecane,
and functionalized electron-neutral alkenes such as 5-bromo-
pent-1-ene, indicating that the first shift of the double bond is
barely reversible in nonpolar benzene and that no other
positional isomers are being formed (entries 7–10, Table 2).
Employing vinyl cyclohexane as a substrate (entries 11–12,
Table 2) showed that isomerization of a terminal double bond
to a sterically demanding tri-substituted position is possible,
although it proceeds slowly even in MeÀ THF, resulting in 87%
conversion after 24 hours. In benzene only 20% conversion was
achieved over the same period of time. However, the double-
bond isomerization into the ring was not observed in this case.
This trend was even more pronounced when functionalized
alkenes such as methyl-4-pentenoate and 4-pentenoic acid
were employed (entries 13–16, Table 2). For these substrates,
exclusive formation of the single-step shifted double bonds in
benzene versus a 1:1 mixture in MeÀ THF was observed. The
oxygen functionalities of these substrates may chelate and they
favour the formation of six-membered metallacycles where the
substrate is prearranged to eliminate the kinetic product.
However, stronger chelating functional groups, such as amines,
slow down or even inhibit the reaction (entries 17–20, Table 2).
Mechanistic rationale. Mechanism-wise, alkene isomerization
reactions operate either via metal-alkyl or π-allyl intermediates
product peaks appeared at 6.4, 6.0, and 1.7 ppm in a 1:0.3:0.6
ratio (C1:C2:C3, respectively, Equation 1).
ð1Þ
This result was somewhat surprising, because the metal-
alkyl pathway either leads to the formation of β-methylstyrene
monodeuterated at the α-position, if the reaction is irreversible,
or to the statistical redistribution of the deuterium atoms along
[
22]
the carbon chain, if the reaction is highly reversible.
Alternatively, under the π-allyl mechanism, no deuteration at
[23]
C2 is expected. The only good explanation for the migration
of the D-atom from C1 and its equal distribution between the
C2 and C3 positions is that the product-forming stage is
essentially irreversible. Here, the IrÀ D species, which is expelled
at the irreversible stage, participates in the reversible migratory
insertion/β-H elimination events that affect only the terminal
double bond and, subsequently, only the C2 and C3 positions
(Scheme 3).
It is reasonable to assume that the degree of reversibility of
the migratory insertion/β-H elimination reactions largely de-
pends on the alkene ability to form a fairly stable complex with
the catalyst. As we demonstrated previously, the metal center
in 4 is coordinatively and electronically saturated. Therefore,
[18]
(
Scheme 2).
The metal-alkyl pathway (A) starts with the
coordination of the alkene to a transition metal-hydride species,
[19]
followed by its insertion into the polar MÀ H bond.
This
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Table 2. Scope and limitation studies.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[
c]
Entry
Substrate
T
Solvent
Conv. [%]
Products
[a]
1
°
80 C
MeÀ THF
>99
Allyl benzene
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
[
a]
2
80°C
80°C
Benzene
74
[a]
3
MeÀ THF
>99
Eugenol
[
a]
4
80°C
80°C
Benzene
>99
[a]
5
MeÀ THF
95
1
-octene
[
[
a]
a]
6
80°C
80°C
Benzene
61
90
7
MeÀ THF
1
-dodecene
[
[
a]
a]
8
80°C
Benzene
62
9
80°C
80°C
MeÀ THF
>99
mixture of compounds
[
b]
a]
10
Benzene
52
[
1
1
80°C
MeÀ THF
87
Vinyl cyclohexane
[a]
1
2
3
80°C
80°C
Benzene
20
[a]
1
MeÀ THF
>99
Methyl-4-pentenoate
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Table 2. continued
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
[
c]
Entry
Substrate
T
Solvent
Conv. [%]
Products
[
a]
1
4
80°C
Benzene
>99
[a]
1
5
80°C
MeÀ THF
94
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
4
-pentenoic acid
[
a]
16
80°C
80°C
Benzene
55
44
[a]
1
7
MeÀ THF
[
[
[
a]
a]
a]
18
80°C
80°C
80°C
Benzene
MeÀ THF
Benzene
5
nd
nd
nd
1
9
<1
<1
2
0
À 3
À 3
Conditions: [a] alkene (0.28 mmol), catalyst 4 (1.4*10 mmol) in 1 mL of MeÀ THF or benzene at 80°C; [b] alkene (0.28 mmol), catalyst 4 (2.8*10 mmol) in
1
ml of MeÀ THF or benzene at 80°C; [c] The yield was determined by NMR spectroscopy using internal standard.
Scheme 3. Isomerization of the D-labeled allyl benzene by stoichiometric 4.
dissociation of the methoxyl group is required for the initial
substrate coordination (Scheme 4).
Thus, the hemilabile sidearm acts as a selector favoring the
less sterically hindered substrates. This ensures the coordination
of the terminal double bonds and prevents recombination and
further isomerization of the internal alkenes, which explains
both the isotope labeling experiment as well as the regiose-
lectivity of the reaction toward the one-step isomerization of
the terminal alkenes. Indeed, isomerization of cis-2-octene by 4
in MeTHF at 80°C for 24 h resulted in only ca. 25% conversion
Scheme 4. Plausible mechanism underlying the alkene isomerization catalyzed by 4.
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0
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3
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1
2
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7
8
9
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
Figure 3. Optimized structures of isomers for complex 4 and their relative energies (in kcal/mol) in benzene and THF.
to 3-octenes versus almost a complete conversion of 1-octene
under the same conditions (entry 5, Table 2). The regioselectiv-
ity in the isomerization of cis-2-octene also matches the results
obtained with the terminal alkenes under these conditions.
Finally, to validate the solvent effect, we performed brief
DFT calculations aiming at evaluating the dissociation of the
MeO-sidearm in THF and benzene. The geometry optimization
at the B3PW91/def2-TZVP theory level performed in the
corresponding solvent indicated that the dissociation-rotation
could induce a selective one-step shift of the terminal double
bond in a broad scope of substrates. The reactivity and the
regioselectivity of the catalytic isomerization were found to be
strongly solvent dependent. Mechanism-wise, the hemilabile
sidearm acts as a selector favoring the less sterically hindered
substrates, which favors the coordination of terminal double
bonds and prevents recombination and further isomerization of
the internal ones. However, the most important conclusion
from this work is that the unique topology of this family of
compounds and flexibility in their synthesis offer nearly
unlimited opportunities for the fine-tuning of the catalytic
activity in many classical reaction schemes, as well as render
new reactivities to many classical organometallic catalysts.
of the CH OMe group is easier in polar solvents. In THF the
2
species with an open coordination site (Figure 3, right) is only
3
kcal/mol higher than the hexacoordinated isomer. The energy
cost of CH OMe group dissociation is twice as large in benzene.
2
The Gibbs free energy difference of two isomers in two solvents
suggests that a tenfold higher content of a catalytically active
pentacoordinate isomer exists in THF.
Acknowledgments
It is also reasonable to suggest, that upon CH OMe group
2
dissociation, complex 4 maintains the fac-conformation of PC
This research was supported by the ISF (Israel Science Foundation)
Grant Nos. 917/15, the RUDN University Program “5-100” and by
the Esther K. and M. Mark Watkins Chair for Synthetic Organic
Chemistry. The DFT calculations and spectroscopic studies were
3
(
sp )ligand, which is more suitable for alkene insertion into the
IrÀ H bond. This step is energetically downhill for a fac-isomer
[24]
1
31
1
(
please, see ESI). Indeed, variable-temperature H and P{ H}
o
NMR spectra show only minor signals of the mer-isomer even in
the presence of 1-octene (see ESI). This may explain the activity
of complex 4 surpassing that of other catalysts that feature the
supported by the Russian Science Foundation (grant N 19-13-
00459). The use of the equipment of the Center for molecular
composition studies of INEOS RAS supported by the Ministry of
Science and Higher Education of the Russian Federation is
gratefully acknowledged. The authors declare no competing
financial interests.
3
mer-configuration of PC(sp )ligand. The different ratio between
2
-alkene and higher regioisomers in the two tested solvents
correlates well with the overall conversion of the reactions.
Conclusions
Conflict of Interest
In conclusion, we studied the isomerization of terminal alkenes
catalyzed by a series of iridium complexes possessing PC(sp )1–
The authors declare no conflict of interest.
3
4
equipped with a hemilabile side-arms. Complex 4, featuring a
more flexible coordination, displayed excellent reactivity and
ChemCatChem 2020, 12, 1–8
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© 2020 Wiley-VCH GmbH
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Full Papers
doi.org/10.1002/cctc.202001308
ChemCatChem
Keywords: pincer complexes · ligand-metal cooperation ·
isomerization · alkenes
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Manuscript received: August 11, 2020
Revised manuscript received: September 7, 2020
Accepted manuscript online: September 8, 2020
Version of record online: ■■■, ■■■■
ChemCatChem 2020, 12, 1–8
www.chemcatchem.org
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© 2020 Wiley-VCH GmbH
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These are not the final page numbers!

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FULL PAPERS
S. De-Botton, D.S. O. A. Filippov,
Prof. E. S. Shubina, Prof. N. V.
Belkova*, Prof. D. Gelman*
1
– 8
Regioselective Isomerization of
Terminal Alkenes Catalyzed by a
PC(sp )Pincer Complex with a
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Hemilabile Pendant Arm
Bifunctional pincer complexes: We
describe an efficient protocol for the
divergent regioselective isomerization
of terminal alkenes employing a bi-
functional Ir-based PC(sp ) complex
possessing a hemilabile sidearm.
3